Episode 7 — DSA with JavaScript / 7.3 — Arrays

7.3 — Quick Revision: Arrays

<< Overview

Array operations complexity

OperationTime
Access by indexO(1)
Push / pop (end)O(1)
Unshift / shift (front)O(n)
Insert / delete at indexO(n)
Linear searchO(n)
Binary search (sorted)O(log n)

Key algorithms

AlgorithmTimeSpacePattern
Kadane's (max subarray)O(n)O(1)Running max
Two-pointer pair sumO(n)O(1)Sorted array
Dutch National FlagO(n)O(1)3-way partition
Prefix sumO(n) build, O(1) queryO(n)Cumulative sum
Sliding windowO(n)O(1)Fixed/variable window
Rotation (3 reversals)O(n)O(1)In-place

Array diagram reference

Index:   0   1   2   3   4
       ┌───┬───┬───┬───┬───┐
Array: │ a │ b │ c │ d │ e │
       └───┴───┴───┴───┴───┘

Two pointers:
  L →               ← R
  ↓                   ↓
  [1, 2, 3, 4, 5, 6, 7]

Sliding window (size 3):
  ┌─────────┐
  [1, 2, 3, 4, 5, 6, 7]
     ┌─────────┐
  [1, 2, 3, 4, 5, 6, 7]

Prefix sum:
  arr:    [3, 1, 4, 1, 5]
  prefix: [3, 4, 8, 9, 14]
  sum(1,3) = prefix[3] - prefix[0] = 9 - 3 = 6

Common pitfalls

  1. Off-by-one errors (< vs <=)
  2. Modifying array while iterating (use copy or reverse iteration)
  3. Integer overflow in sum calculations
  4. Not handling empty array
  5. Confusing in-place vs new-array methods

Self-check drill

SC-001

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-002

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-003

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-004

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-005

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-006

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-007

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-008

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-009

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-010

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-011

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-012

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-013

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-014

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-015

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-016

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-017

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-018

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-019

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-020

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-021

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-022

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-023

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-024

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-025

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-026

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-027

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-028

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-029

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-030

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-031

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-032

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-033

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-034

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-035

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-036

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-037

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-038

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-039

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-040

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-041

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-042

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-043

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-044

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-045

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-046

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-047

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-048

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-049

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-050

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-051

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-052

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-053

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-054

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-055

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-056

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-057

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-058

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-059

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-060

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-061

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-062

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-063

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-064

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-065

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-066

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-067

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-068

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-069

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-070

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-071

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-072

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-073

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-074

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-075

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-076

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-077

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-078

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-079

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-080

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-081

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-082

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-083

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-084

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-085

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-086

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-087

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-088

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-089

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-090

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-091

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-092

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-093

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-094

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-095

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-096

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-097

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-098

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-099

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-100

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-101

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-102

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-103

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-104

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-105

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-106

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-107

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-108

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-109

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-110

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-111

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-112

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-113

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-114

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-115

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-116

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-117

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-118

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-119

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-120

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-121

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-122

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-123

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-124

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-125

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-126

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-127

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-128

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-129

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-130

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-131

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-132

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-133

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-134

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-135

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-136

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-137

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-138

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-139

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-140

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-141

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-142

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-143

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-144

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-145

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-146

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-147

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-148

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-149

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-150

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-151

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-152

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-153

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-154

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-155

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-156

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-157

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-158

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-159

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-160

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-161

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-162

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-163

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-164

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-165

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-166

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-167

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-168

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-169

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-170

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-171

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-172

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-173

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-174

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-175

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-176

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-177

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-178

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-179

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-180

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-181

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-182

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-183

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-184

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-185

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-186

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-187

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-188

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-189

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-190

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)

SC-191

  • Q: What is array access time complexity?
  • A: O(1) — index-based direct calculation

SC-192

  • Q: Explain Kadane's algorithm in one sentence.
  • A: Track max of (extend current subarray or start new) at each element

SC-193

  • Q: What is the two-pointer technique?
  • A: Two indices move based on conditions, avoiding nested loops

SC-194

  • Q: How to rotate array in O(n), O(1) space?
  • A: Three reversals: full, first k, rest

SC-195

  • Q: What is a prefix sum used for?
  • A: O(1) range sum queries after O(n) preprocessing

SC-196

  • Q: What does Dutch National Flag solve?
  • A: Sort array of 0s, 1s, 2s in single pass with 3 pointers

SC-197

  • Q: How to find missing number in 1..N?
  • A: Sum formula n(n+1)/2 minus array sum, or XOR all values

SC-198

  • Q: What is sliding window?
  • A: Fixed-size window slides, maintaining running sum/state

SC-199

  • Q: Time complexity of push vs unshift?
  • A: push is O(1) amortized, unshift is O(n) due to shifting

SC-200

  • Q: Boyer-Moore voting — what does it find?
  • A: The majority element (appears > n/2 times)